Darkness 2 for PC is on sale via amazon for $25... BUT you can choose to use the code and unlock it/download via Steam. Same thing for Darksiders ($5). Bought both last night, and can confirm unlocking on Steam worked perfectly. Didn't know this was an option on amazon so I wanted to get the word out.
It's also worth saying it seems only specific games on amazon can be unlocked on steam this way. For example Witcher 2 is $15 but it doesn't seem like it will unlock on steam so I passed.